The invention relates to a molded case electrical circuit breaker, comprising at least one connection terminal to a conductor designed to be protected by the circuit breaker, detection means of the current flowing in the conductor, measurement means of the temperature in a predetermined zone of the circuit breaker, an electronic trip device, to which the output signals from the current detection means and the temperature measurement means are applied, performing at least a delayed tripping function so as to generate a tripping order when the current exceeds preset thresholds, the trip device comprising means designed to modify the time delay in terms of the temperature measured when the circuit breaker closes, the circuit breaker comprising tripping means controlled by said tripping order.
Conventional electronic trip devices do not take account of the thermal history of the system which has been disconnected by the associated circuit breaker when the circuit breaker recloses. This may, in some cases, lead to a dangerous temperature rise of the system. In practice, conventional electronic trip devices simulate the thermal state of the system mathematically in the event of a short or long delay fault, the time delay then being inversely proportional to the square of the fault current. This partial simulation of the thermal state of the system only takes account of the temperature rise due to the fault current and of the cooling due to a possible decrease of the fault current during the time delay period. The flow chart in FIG. 1 schematizes the long delay function of a conventional microprocessor-based trip device. A quantity T.degree.LR, which is the thermal image representative of the thermal state of the system protected by the circuit breaker, is initially set to a minimum value, 0 in the figure. The measured phase current I, in fact the highest current flowing in one of the phases of the system protected by the circuit breaker, is compared with an adjustable long delay threshold S. When this current is greater than or equal to the threshold S, the quantity T.degree.LR is replaced by the sum T.degree.LR+I.sup.2, thus simulating the temperature rise of the system. The new value T.degree.LR, thus computed, is compared with a new adjustable maximum value T.degree.LRmax. So long as this maximum value is not reached the process continues, thus resulting in a time delay inversely proportional to the square of the fault current. When the maximum value is exceeded the trip device produces a tripping order of the associated circuit breaker. If the current is lower than the threshold S, the quantity T.degree.LR is decremented, for example replaced by T.degree.LR.times.e.sup.-t/.tau., .tau. being a preset cooling time constant and t the time elapsed since the fault ceased, thus simulating the cooling of the system. When T.degree.LR reaches the minimum value, 0 in the figure, the quantity T.degree.LR remains unchanged so long as the current I remains below the threshold S. Thus, if the conditions required to bring about tripping disappear before tripping occurs, there is no tripping order and a return to normal takes place progressively so as to simulate a thermal memory of the system.
It is moreover advisable, when a circuit breaker recloses, to take account of the thermal state of the system which has been disconnected by the circuit breaker. This thermal state depends both on the thermal state existing just before the trip and on the time elapsed since the trip, during which time the system has cooled down.
It has been proposed (U.S. Pat. No. 4,616,324) to continue the simulation of cooling after a circuit breaker trip using an RC circuit charged at the time of tripping.
Such an approach does not however take account of all the factors involved in the system temperature rise and in many cases the thermal image thus obtained is not representative of the actual state of the system.
It has moreover been proposed (U.S. Pat. No. 4,631,625) to initialize the tripping characteristics of an electronic trip device, when the circuit breaker closes, according to the value of thermistors located on the conductors protected by the circuit breaker. Locating thermistors on the conductors gives rise to problems, notably of fixing and insulation, and the temperature of the conductors is not in fact truly representative of the system temperature rise.
The object of the present invention is to achieve a circuit breaker with an electronic trip device enabling the thermal history of the system to be protected to be better taken into account.